The invention relates to a method for converting electromagnetic energy.
The invention also relates to an apparatus and component suited for converting electromagnetic energy.
Conventionally, a barcode is most generally used for identification of objects. The barcode is generally read by means of a laser beam. Also passive/active code circuits or tag memories operating in the RF range have been developed. These components operate either in the low-frequency range (at about 100 kHz), medium-frequency range (at about 13 MHz), high-frequency range (at 400 MHz) or at the microwave range (at 2.46 GHz or 5.4 GHz). Most of the identification code or tag memory circuits are based the use of a xe2x80x9ctraditionalxe2x80x9d memory circuit. The circuit is energized from the scanning electromagnetic field. Also SAW (Surface Acoustic Wave) circuits have been employed at high frequencies. In a prior-art method of this type, the electrical pulse is converted into a surface acoustic wave that is reflected from metallic reflector elements fabricated on the surface of a piezoelectrically active material. Such a circuit is rather expensive to produce and the response effect is linear which makes remote identification of the circuit nonideal. Furthermore, it is difficult to tune the circuit so that the identification information covers a narrow frequency band.
A problem hampering the use of passive identification circuits based on conventional memory techniques is the limited detection range thereof. If low-frequency magnetic fields are employed, the field strength falls very rapidly (as the third power of distance). The voltage induced at the circuit may be increased by means of tuned circuits or transformers, but even this approach makes it extremely clumsy to extend the detection range up to 0.5 m. In contrast, microwave technology allows easy focusing of the scanning beam, whereby relatively large field strengths can be evoked even at larger distances. Unfortunately, it is difficult to generate a substantially high supply voltage by means of high-frequency fields. It must be noted, that the problem is not principally related to the transmission of a sufficient power, but rather, how to generate a sufficiently high supply voltage level for silicon microcircuits. Today, it is a generally established rule of thumb that a supply voltage of about 3 V cannot be generated at a distance of 10 m, which means that commercial firms active in the field are looking for other solutions to implement an identification transponder. The use of an SAW circuit is one example of such attempts.
Besides those described above, microwave power can be detected by means of a bolometer. While a bolometer is well suited for use in conjunction the short-wavelength bands of the electromagnetic spectrum, at still shorter wavelengths, e.g., at the microwave and millimeter wavelengths, the noise of the required front-end electronics becomes a definitely limiting factor.
It is an object of the present invention to overcome the disadvantages of the above-described techniques and to provide an entirely novel type of method and apparatus for converting electromagnetic energy.
The goal of the invention is achieved by virtue of using such a transponder component that comprises electromechanical vibrating elements such as beam capacitors that with the help of, e.g., an inductor, are tuned to a desired resonant frequency. Additionally, such a component includes means for converting mechanical energy back to electrical energy at a different voltage and/or frequency in regard to that of the electrical signal received by the component.
A preferred embodiment of the invention is based on the use of such a passive identification component that comprises electromechanical vibration elements, e.g., beam capacitors that are tuned to a desired resonant frequency with the help of an inductor, for instance. According to another preferred embodiment of the invention, a number of the capacitors can be electrically set into a passive state in order to provide each identification component with an identification code different from that of any other similar component.
A further preferred embodiment of the invention is suited for power measurement on the microwave and millimeter wave ranges and, for this purpose, is based on power measurement of an electromagnetic field by means of a micromechanical transducer, generally a silicon micromechanical transducer. Typically, the micromechanical transducer is made using a beam structure, and the mechanical state of the micromechanical structure is altered by means of electrical bending forces in order to increase its sensitivity and to tune the frequency range so that the measurement range can be changed. The micromechanical transducer is tuned either to the measured frequency or, alternatively, between the receiver antenna and the circuit containing the micromechanical transducer is advantageously adapted a chopper that switches on and off the microwave signal, advantageously at the resonant frequency of the mechanical circuit structure.
More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.
Furthermore, the apparatus according to the invention is characterized by what is stated the characterizing part of claim 1.
The invention provides significant benefits.
A component made using surface micromechanical machining is cost-efficient to manufacture. In the embodiment according to the invention, the transmit and receive frequencies are arranged to be different from each other, which allows easy separation of the return signal from the transmitted signal. According to the invention, also the coding of the identification component can be made readily carried out using electrical means. The invention allows radio-frequency energy to be converted into such a form of electrical energy that is compatible with the voltage requirements of circuits fabricated using the silicon semiconductor technology.
By virtue of the invention, it is possible to design a mechanical element of a high quality factor (Q-value) for microwave power detection, whereby a clearly superior sensitivity over prior-art techniques particularly in the detection of field strengths at millimeter and microwave ranges is achieved. Moreover, the center frequency of the novel detector can be altered without any essential degradation of its quality factor.
If an RF field is modulated at a given frequency and this modulation exerts at the modulating frequency a force capable of evoking a micromechanical resonant vibration, the amplitude of vibration at the resonant frequency can be made so large that the associated electronic circuits becomes a nonlimiting factor to the noise, which will rather be determined by such variables as, e.g., damping of the micromechanical resonator by the surrounding gas atmosphere. It can be proven by calculations that micromechanical machining makes it possible to manufacture for the millimeter wave range (or equivalent RF range) a detector of a higher sensitivity than that offered by a bolometer.